A delta-sigma modulator in a high precision delta-sigma analog-to-digital converter (ADC) is designed to add an analog input signal with a reference voltage at a summing node to generate an analog difference signal. The analog difference signal is integrated and quantized to produce a digital output signal. The digital output signal is then coupled to a digital-to-analog (D/A) converter which selects a reference voltage level between positive V.sub.REF+ and negative V.sub.REF- sources, to apply to the summing node in the next sampling period. The accuracy of the A/D or D/A conversion depends on the accuracy of the reference voltage level.
In a delta-sigma modulator integrated circuit (IC), the reference voltage sources have some measurable impedance since the production of reference voltages with a virtually zero source impedance over the signal frequency range is impractical and cost-prohibitive. If loading on the reference voltage varies with the analog input signal, then the delta-sigma modulator has non-linear errors due to this signal-dependent loading of the reference voltages.
Delta sigma modulator ICs generally have external anti-aliasing networks connected to the input pins. These networks are typically RC filters. Any signal-dependent current drawn by the input pins will cause a voltage drop across the resistance R of the RC filter, thereby causing a gain error.
Thus, what is needed is a delta-sigma modulator that has signal-independent loading of the voltage reference and signal-independent input current.